The present invention relates to a transparent shrink film made from polypropylene useful for all-round labeling of essentially cylindrical containers such as bottles. The invention furthermore relates to a process for the production of this type of film.
Presently, the following two main processes for all-round labeling of essentially cylindrical containers or bottles are known:
1. A flat film is first converted into a tube or into tube sections, which is hen inverted over the bottle and subsequently shrunk onto the bottle in a shrink oven. PA0 2. The film is processed directly from the roll, from which the film is first wound around the bottle/container and then bonded in an overlapping manner. In the subsequent shrink oven treatment, the film comes into close contact with the container.
For the first-mentioned process, a film having high shrinkage perpendicular to the longitudinal axis of the bottle is required; this direction is identical to the transverse direction of the film. For the second process, a film having increased shrinkage in the direction of winding is required; this means that shrinkage in the longitudinal direction is preferred.
The present invention relates to a film which is especially suitable for use in the second process. The instant film simultaneously achieves high longitudinal shrinkage and .slow transverse shrinkage throughout the relevant temperature range.
At present, films useful in the second process usually comprise polyvinyl chloride or polystyrene and are generally stretched longitudinally. Both of these materials have disadvantages. Specifically, the relatively high price due to the high density of the polyvinyl chloride, and the high formulation price for the glass-clear form of the polystyrene.
Other materials, for example biaxially oriented polypropylene, have hitherto not been employed to any great extent for the above application. In the case of biaxially oriented polypropylene, it is theoretically possible to achieve the desired shrink properties in the longitudinal direction by using only longitudinal stretching. However, this film has the disadvantage of poor mechanical properties. A relatively large film thickness is therefore necessary for processing on fast-running machines such as at 600 units/minute, which is customary today. The relatively thick film is relatively cloudy and non-glossy, which adversely affects the appearance. In addition, a purely longitudinally stretched film tends to split in the longitudinal direction, an effect which is extremely undesired during processing. In addition, a longitudinally stretched film also has the general property of expanding in the transverse direction, i.e., in the direction perpendicular to the shrink direction, under the action of heat. This property can result in the label projecting beyond the edge of the container.